Exploring green environmental composites as hosts for shielding materials using experimental, theoretical and Geant4 simulation methods

Rice straw is considered an agricultural waste harmful to the environment, which is abundant in most parts of the world. From this point, the present study is devoted to preparing new composites of two types of glue based on rice straw as a plentiful, low-cost matrix. Straw glue samples were prepared by mixing 20% wt. of rice straw with 80% wt. of animal glue (RS-An) and polyvinyl acetate (RS-PVAC) at different thicknesses of 1, 2, and 3 cm. The chemical composition of the prepared samples was identified by energy dispersive X-ray analysis and their morphology was examined using a scanning electron microscope. The mechanical test explored that RS-An and RS-PVAC respectively required a stress of 25.2 and 25.5 MPa before reaching the breaking point. γ-ray shielding performance was analyzed and determined at numerous photon energies from 0.059 to 1.408 MeV emitted from five-point γ-rays sources using NaI (Tl). Linear attenuation coefficient was calculated by obtaining the area under the peak of the energy spectrum observed from Genie 2000 software in the presence and absence of the sample. The experimental results of mass attenuation coefficient were compared with theoretical data of XCOM software with relative deviation ranging from 0.10 to 2.99%. Geant4 Monte Carlo simulation code was also employed to validate the experimental results. The relative deviation of XCOM and Geant4 outcomes was 0.09–1.77%, which indicates a good agreement between them. Other radiation shielding parameters such as half value layer (HVL), tenth value layer, and mean free path were calculated in three ways: experimentally, theoretically from the XCOM database, and by simulation using Geant4 code. Additionally, effective atomic number (Zeff), effective atomic number (Neff), equivalent atomic number (Zeq), and buildup factors were evaluated. It was confirmed that the γ-ray shielding properties were further boosted by mixing rice straw with the animal glue compared to the synthetic one.


Material
Rice straw was collected from rice mills in some agricultural areas in Egypt.Then it was sieved to get rid of any impurities.The collected straw was crushed and ground into very small pieces by a blinder.Animal and white glue were imported from the local market.

Sample preparation
Straw-glue samples, shown in Fig. 1, were prepared by adding 20% straw to 80% glue material.The combination of straw and animal glue (RS-An) and straw and polyvinyl acetate (RS-PVA C ) was thoroughly mixed until a uniform paste was achieved.This mixture was then poured into cylindrical molds with a diameter of 8 cm and varying heights of 1 cm, 2 cm, and 3 cm.The molds were left to dry naturally in the open air.This process ensures the formation of solid composite blocks with consistent composition and dimensions.
The Archimedes technique, according to ASTM D 792-9124, was applied to calculate the density of each composite sample by using Eq. ( 1) 27 .For this purpose, a calibrated electrical balance with a single pan of accuracy of 0.0001 g and ethanol as an organic liquid was used.
where M is the mass of the sample and V is its volume.The measured densities of RS-An and RS-PVA C samples are 0.84767 and 0.73862 g/cm 3 , respectively

Morphology
The scanning electron microscope (SEM) analysis was conducted using the JSM-5300, JEOL type.The samples were coated using an ion sputtering coating device and then placed inside the electron microscope unit with an operating voltage of 20 keV, and the magnification order was 1000×.

Mechanical measurements
As shown in Fig. 2, the cylinder specimens of diameter 22 mm and thickness 22 mm were cast to determine compressive strength.The mechanical measurements were conducted at room temperature using the universal testing machine (UTM), and the compressive strength was measured at a rate of 50 mm/min.

Gamma-ray spectroscopic setup
The shielding parameters were calculated in an experiment that measured the intensity of the γ-rays passed through the specimen.The detector used in this work was a NaI(Tl) scintillation detector 28 .Figure 3 shows the configuration for this study, where the source-detector distance was 45 cm and the detector-sample distance was 4 cm.Five standard radioactive point sources are used as follows: Am-241, Co-60, Ba-133, Cs-137, and Eu-152 to emit a broad spectrum of energies ranging from 0.059 to 1.408 MeV, as listed in Table 1.The emitted photons interact with the detector crystal and transform into electrical signals that can be analyzed using Genie 2000 software.Figure 4 shows an example of the Cs-137 spectrum obtained in the absence of absorbers.

Shielding parameters
The area under the photo-peak, related to used radioactive sources, was calculated in the presence and absence of the sample, which represents the transmitted (I) and initial (I 0 ) intensities, respectively.Calculating the LAC is the first step to evaluating the material capability for shielding.LAC can be determined using Beer-Lambert's law 29 : where t is the thickness of the absorber.
The MAC is measured by dividing the LAC for a given material by its density (ρ), as represented in Eq. (3) 30 : (2)  The theoretical values of MACs for the given samples were calculated using XCOM software Code.Additionally, the relative deviation (Δ%) between the experimental and theoretical values of MACs at the investigated photon energies is listed in Table 2.The Δ% is evaluated according to the expression 31 : The HVL is a crucial parameter when making an appropriate radiation-protecting substance.It is defined as the absorption thickness needed to decrease the incident radiation on the substance to 50% of its initial value and is calculated by Eq. (5) 32 :  www.nature.com/scientificreports/Also, the thickness at which the gamma-ray photon intensity passes through the material decreases to a tenth of its initial value, called TVL, and is given by 33 : MFP is defined as the distance between two successive interactions; mathematically, it is the inverse of the LAC.The MFP was calculated for shielding material using the following Eq.( 7) 34 : The effect of the chemical composition of a shielding material is always clarified using the effective atomic number (Z eff ), and its variation with energy may be used to investigate the relative changes in photon absorption processes with energy for various shields.Z eff was calculated from MAC based on Eq. (8) 35 : where A i and f i are the atomic weight and the molar fraction of the i th constituent element in the composite, respectively.
The effective electron density (N eff ), measured in electrons/g, defined as the number of electrons per unit mass of the material, is derived using the calculated Z eff according to Eq. ( 9) 36 : where i f i A i represents the mean atomic mass of the sample, and N A is the Avogadro's number.
A primary factor used for designing a radiation shielding medium is the buildup factor (BF). BFs are divided into two types: the energy absorption buildup factor (EABF) and the exposure buildup factor (EBF).BFs were obtained by computing them from the Geometrical Progression (G-P) fitting method at energies ranging from 0.015 to 15 MeV.To calculate BFs, firstly, it is important to compute the equivalent atomic number (Z eq ) of the composite using Eq. ( 10) 37 : where R is the ratio of MAC Compton /MAC Total for the composite at a given energy.Also, Z 1 and Z 2 are atomic numbers of elements according to ratios R 1 and R 2 , respectively.The calculated Z eq values of the investigated materials were then used to interpolate the GP fitting parameters (b, c, a, X K , and d) in the range of energy between 0.015 and 15 MeV using the interpolation formula: where C 1 and C 2 are GP fitting parameters, taken from the ANSI/ANS-6.4.3 standard database 38 .
Finally, the BFs for the selected samples were then estimated with the help of the obtained GP fitting parameters using the following relations 39 : where K(E,x) is the photon dose multiplication factor, which is obtained by: where x is the mean free path (MFP) that ranged from 1 to 40 mfp (1 ≤ x ≤ 40).

Geant4 simulation
Geant4 is a widely used object-oriented toolkit for simulating the transport of charged and neutral particles through matter 40 .Geant4 is based on Monte Carlo techniques, which are particularly well-suited for simulating complex particle interactions in matter 41 .In this study, we used the Geant4 toolkit to simulate the transport of photons through two different composite materials, RS-PVAc and RS-An, in the energy range of 0.059-1.408MeV.The simulation aimed to measure the linear attenuation coefficients (LACs) of the two composites, from which other shielding parameters, including HVL, TVL, and MFP, can be calculated.The simulation was performed using Geant4 version 11 on a Linux operating system, using a single cubic box of (6) TVL = ln 10 LAC  2. To simulate the LACs, we randomly projected 10 6 main monoenergetic events, labeled as (I 0 ), at the material's edge as parallel rays.The Monte Carlo simulation uses this large number of incident events to minimize statistical error.During the simulation, the substance may either absorb or transmit every incident photon.The number of photons that dissipate energy due to the three primary interactions (photoelectric effect, Compton scattering, and pair production) and those transmitted in the absence of any interaction can be calculated at the end of the simulation.Using Eq. ( 2), LAC values for each composite can be determined, where (I) represents the total number of photons transmitted at the end of the simulation.The Geant4 simulation setup used in this study is illustrated in Fig. 5.

Composition analysis
The composition of the prepared samples was analyzed using an Energy Dispersive X-ray (EDX) spectrometer, and the results have been documented in Table 2.As shown in Fig. 6, the presence of nitrogen in the RS-An sample with a percentage of 13.99% and its absence in the RS-PVA C sample.Furthermore, Fig. 7 shows that the percentage of calcium in RS-PVA C was found to be lower than that in RS-An.

Scanning electron microscope
The morphology of the prepared samples was analyzed by SEM to study their microstructure.SEM-EDX of the selected samples gave microanalysis data on their qualitative and semi-quantitative chemical composition.
Based on the SEM micrograph, it is evident that the pores are placed very closely to each other, resulting in a  www.nature.com/scientificreports/decrease in photon transmission.Therefore, it leads to attenuating the photons within the randomly distributed particles, further impeding their propagation 42 .Figure 8 depicts the clear visibility of the pores of rice straw, with image 8-a showing that the pores of the straw were significantly filled and covered with polyvinyl acetate glue.Conversely, in the sample containing animal glue, image 8-b indicated that animal glue covered the pores to a lesser extent.This can be attributed to the higher viscosity of PVA C glue than animal glue.Furthermore, it confirmed the excellent mixing of the samples.

Mechanical testing
Figure 9 depicts the stress-strain curves for RS-An and RS-PVA C samples.As seen in Fig. 9, the RS-PVA C sample endured an applied force of 28.5 MPa before reaching its breaking point.In comparison, the RS-An sample reaches its breaking point at an applied load of approximately 28.2 MPa 43 .

Shielding parameters
The MACs in the energy region between 0.059 and 1.408 MeV were measured experimentally, theoretically using XCOM software, and by Geant4 simulation code.The comparison between experimental, theoretical, and Geant4 simulation results for the investigated samples is presented in Table 3.The relative deviation (Δ%) between the experimental and XCOM values of MACs at the examined photon energies varies between 0.10 and 2.99%.This close agreement validates the experimental technique using a narrow beam and thin absorber with the theoretical values.Furthermore, the relative deviation (δ%) between the XCOM and Geant4 simulation values of MACs and the relative deviation (RD%) between the experimental and Geant4 values of MACs showed good agreement at all the selected energies.The results demonstrated that the simulated values of the MACs using the Geant4 code are very close to those resulting from the XCOM database and experimental measurements.It is evident that there is a good agreement between the experimental, theoretical, and simulation results.Figure 10 shows that the LAC values decrease as energy increases.The maximum value of LAC at lower energy (0.059 MeV) is 0.1507 cm −1 , which belongs to RS-An, and 0.1395 cm −1 for the RS-PVA C sample.Also, LAC is 0.0649 cm −1 and 0.0573 cm −1 at moderate energy (0.6617 MeV), and at higher energy (1.408 MeV), is 0.0445 cm −1 and 0.0394 cm −1 for the RS-An and RS-PVA C samples, respectively.It is observed that at photon energies ranging from 0.059 to 0.121 MeV, the LAC decreases sharply as the photon energy increases in this range.This is because at energies lower than 0.125 MeV, the cross-sections for the photoelectric interactions are sufficiently high, which depends on Z 4 /E 3.544 , where Z is the atomic number of the absorbing element, and E is the photon energy.Moreover, as the photon energy increases to exceed 0.121 MeV, the LAC of each composite slightly decreases with increasing photon energy.This is because at this intermediate energy range, the effect of photoelectric absorption decreases, and Compton scattering becomes the dominant mechanism.As expected, the higher the gamma-ray energies, the  higher the penetration properties due to the dominance of the pair production process above 1.22 MeV. Figure 10 demonstrates that the RS-An composite is better at attenuating gamma rays than the RS-PVA C composite.HVL and TVL parameters are more practically used to describe a material's shielding capability against photons and for designing practical shields.HVL is the thickness of the samples required to reduce incident photon intensity by one-half.The shielding effectiveness of the sample is inversely proportional to its HVL value.In Fig. 11, the results of HVL values plotted against photon energies show that HVL values increase with increasing photon energies.The experimental results revealed that HVL decreased from 4.968, 12.096, and 17.592 cm for RS-PVAC to 4.599, 10.680, and 15.576 cm for RS-An samples at photon energies of 0.059, 0.6617, and 1.408 MeV, respectively.Also, Fig. 11 shows the agreement between XCOM values and simulated values by Geant4 code.Similarly, Fig. 12 displays the investigated samples' TVL experimental, theoretical, and Geant4 values.It is worth noting that there is a commendable concurrence between the Geant4 simulated data and the theoretical data.Figure 12 indicates that the TVL values rise as the energy increases, highlighting the greater efficacy of the RS-An sample over the RS-PVA C sample.
Figure 13 illustrates the variation of the experimental, theoretical, and Geant4 simulated values of MFP of the studied samples among different photon energies.The MFP values increase as gamma-ray energy increases.When the MFP values are low, there is a higher likelihood that gamma rays will be absorbed or weakened as they travel through a medium 45 .Obviously, in the RS-An sample, the photon loses its energy at a shorter distance than in the RS-PVA C sample.For all studied models, it can also be noted that there is a close match between the experimental, theoretical, and Geant4 code results.
The Z eff of the present samples was calculated from the determined MACs.The obtained results are displayed graphically in Fig. 14.As can be seen from Fig. 14a, Z eff is a function of the elemental composition of the sample, so the values of Z eff increased as the concentration of heavy elements increased.Figure 14a indicates that the Z eff values vary from 7.60 to 8.58 for the RS-PVA C and from 7.30 to 7.75 for the RS-An sample, as energy varies from 0.059 to 1.408 MeV.From Fig. 14a, it is obvious that the Z eff of RS-PVA C is comparatively higher than that of the RS-An sample.This is increasing mainly due to the higher concentration of element Ca 46 in the RS-PVA C sample.For RS-PVA C and RS-An samples, the relation of the N eff values with the photon energy in the range of 0.059-1.408MeV is presented in Fig. 14b.One can notice that the variations in N eff values are very similar to the trends identified for Z eff values.N eff values vary at 0.059 MeV from 6.53 × 10 25 to 7.34 × 10 25 electron /g and at 1.408 MeV from 6.15 × 10 25 to 6.51 × 10 25 electron/g for RS-An and RS-PVA C samples, respectively.
The Z eq describes the shielding characteristics of the chosen polymers pertaining to equivalent elements and is also considered when determining the buildup factors.The best shielding materials are those with higher Z eq .The Z eq values of the RS-An and RS-PVA C samples as a function of photon energy in the range of 0.015-15 MeV are depicted in Figure 15.As can be seen in Fig. 15, the RS-An sample has the lowest Z eq values, whereas the RS-PVA C sample has the highest values.Furthermore, it is also apparent that at 1 MeV, the Z eq reaches its maximum value for all composites.This behavior can be illustrated due to the Compton scattering (CS) interactions that dominate in the mid-(γ) energy region.As the Z eq calculations mostly depended on the ratio of (MAC CS /MAC total ), there is likely significant CS in the medium energy zone, which accounts for the greater reported rise in Z eq values.The pair production process then takes over in the higher energy areas, causing Z eq to drop rapidly as the γ-ray energy approaches 1.22 MeV.The dependence of the buildup factors (BFs) of the investigated samples with photon energy between 0.015 and 15 MeV are graphically plotted in Fig. 16 at fixed penetration depth values of 1, 15, 25, and 40 mfp. Figure 16a and b can be separated into three energy regions: the first is the photoelectric absorption region, which is considered the main interaction mechanism at low energies, where BFs of the samples have small values in this energy region.The second region is due to the Compton scattering, which is the predominant interaction at the   EBF values increase until they reach their maximum value and then decrease with increasing energy.Similarly, Fig. 16b shows the same trend for the variation of EABF values with photon energy.This can be discussed in the fundamentals of partial interaction processes.At low energies, EBF and EABF values are the lowest because photons are absorbed at high energies.At intermediate ranging, EBF and EABF values were the largest because photons were degrading by scattering in medium 44,48 .Moreover, the highest BFs values were observed at a deep penetration of 40 mfp while the lowest values were obtained at 1 mfp.The effectiveness of the current work's shielding materials was compared to commonly used materials for gamma ray shielding, including polymer clay, pure high-density polyethylene, recycled high-density polyethylene, poly boron, natural bentonite, and nylon 6.The comparison, presented in Table 4, revealed that the current work's materials exhibited promising attenuation parameters, particularly at low and intermediate energies.These findings highlight the potential of the current work's materials as efficient and environmentally friendly options for gamma ray shielding.

Conclusion
In the current study, two new composites based on a mixture of rice straw, either with animal glue or polyvinyl acetate glue, were prepared and examined as a plentiful and low-cost matrix to be used in radiation shielding applications.The SEM was utilized to study the morphology and distribution of rice straw within the glue.The SEM analysis revealed that the rice straw is mixed and distributed well within either animal glue or polyvinyl acetate glue.The mechanical test revealed that the stress and strain of the RS-An sample were 25.2 MPa and 69.9%, respectively.On the other hand, the RS-PVA C sample showed values of approximately 25.5 MPa and 66.8% for stress and strain, respectively.The MACs of the prepared samples were calculated experimentally and compared to those obtained theoretically using the XCOM program.This comparison proves a good agreement between the experimental and theoretical data.The results demonstrated that the LAC for the RS-An sample was higher than that of the RS-PVA C sample.The HVL and TVL values of RS-An are less than those of RS-PVA C .These results reveal that the RS-An sample is promising for practical applications.Furthermore, the Geant4 Monte Carlo simulation code was also employed to validate the experimental results.The simulated outcomes generated by the Geant4 program were compared to the theoretical results obtained from the XCOM program.It was observed that the Geant4 code accurately simulated real-world data, resulting in a reasonably strong correlation between the practical and theoretical results.Finally, all measurements conducted in this study clearly demonstrate that the rice straw composites serve as a crucial and environmentally friendly solution for radiation protection.The findings highlight the potential of these composites to effectively shield against radiation, contributing to sustainable practices in the field of radiation protection.

Figure 2 .
Figure 2. Rice straw/glue composites: (a) animal glue and (b) PVA C glue for mechanical testing.

Figure 4 .
Figure 4.The acquired spectrum using Cs-137 radioactive source in the absence of absorbers.

Figure 6 .
Figure 6.EDX chart for the RS-An sample.

Figure 7 .
Figure 7. EDX chart for the RS-PVA C sample.

Figure 8 .
Figure 8. SEM images of (a) RS-PVA C and (b) RS-An samples.

Figure 10 .
Figure 10.LAC of experimental, theoretical, and Geant4 simulated values of RS-An and RS-PVA C samples.

Figure 11 .
Figure 11.HVL of experimental, theoretical, and Geant4 simulated values of RS-An and RS-PVA C samples.

Figure 12 .
Figure 12.TVL of experimental, theoretical, and Geant4 simulated values of RS-An and RS-PVA C samples.

Figure 13 .
Figure 13.MFP of experimental, theoretical, and Geant4 simulated values of RS-An and RS-PVA C samples.

Figure 14 .Figure 15 .
Figure 14.The variation of (a) Z eff and (b) N eff of the RS-An and RS-PVA C samples at different energies.

Figure 16 .
Figure 16.The values of (a) EBF and (b) EABF for RS-An and RS-PVA C samples as a function of energy at penetration depths of 1, 15, 25, and 40 mfp.

Table 1 .
Standard radioactive point sources and their emitted photon energies.

Table 2 .
Chemical compositions of the prepend samples.

Table 3 .
Comparison between the experimental, XCOM, and Geant4 values of the MACs of the prepared samples.

Table 4 .
Comparison of the present work with other previously reported shielding materials.